Investigators from the Centers for Disease Control and Prevention conducted a new study with state and local public health departments last summer in which they collected samples from pool filters at 161 pools in the metro-Atlanta area. 58 percent of the samples contained a particular bacterium that lives in the digestive tract of humans and other warm-blooded animals. The researchers treated the presence of the bacteria - which is actually E. coli - as “a fecal indicator,” they wrote in their report.

The CDC suggests that swimmers delivered some of the bacteria into the water by failing to take a thorough shower before getting into the pool. However, larger quantities of E. coli could be introduced through "a formed or diarrheal fecal incident in the water," the report notes.

Pools in private clubs were less likely to contain E. coli, but investigators still found it in 49 percent of cases. Municipal pools had the highest incidence of the bacteria - 70 percent - followed by water parks, at 66 percent.

Investigators also found Pseudomonas aeruginosa prevalent in 59 percent of all pools tested. This bacterium can cause swimmer’s ear, an inflammation of the outer ear and ear canal. It also causes itchy skin.